1. Field of the Invention
The present invention relates to heat-shrinkable films used as materials for shrink labels that are applied typically to containers.
2. Description of the Related Art
Shrink labels containing heat-shrinkable films as base films have been widely used as labels to be applied to containers typically for drinks and condiments. Heat-shrinkable films for shrink labels generally contain polyester resins or polystyrenic resins, and these resins can highly shrink. However, films containing polyester resins have high shrinkage percentages and high shrinkage stress, must be shrunk under severe conditions for beautiful finish and thereby are poor in workability. Films containing polystyrenic resins can easily beautifully finished but have poor dimensional stabilities in storage. Thus, they must be stored under severe control and are poor in handleability. Certain heat-shrinkable films are multilayer films containing layers of different resins, for adjusting film properties such as strength and shrinkability. Examples of such multilayer films are films formed by lamination; and co-extruded films having an adhesive layer. These films, however, have low maximal shrinkage percentages due to the adhesive layer. Therefore, labels containing these films are unsuitable for being applied to portions or dimensions of adherends which must highly shrink, since beautiful finish may not be obtained. In other words, these labels can only be applied to limited portions of the adherends or applied to adherends having limited shapes.
As co-extruded films containing different resins and having no adhesive layer, Japanese Unexamined Patent Application Publication (JP-A) No. 2000-159946 discloses a heat-shrinkable label containing a multilayer film of an amorphous resin layer and an olefinic resin layer, and JP-A No. 07-137212 discloses a heat-shrinkable label containing a multilayer film of a polyester resin layer and a styrenic resin layer. However, the former label has a low interlayer bonding strength, therefore requires an adhesive layer in actual use, and, in addition, fails to achieve a high shrinkage percentage. The latter film is hard and has low flexibility, and the resulting label after shrinkage may often invite problems such as cracking. In addition, containers, typically as PET bottles (poly(ethylene terephthalate) bottles) with the latter label are unsuitable for recycling by separating and recovering the containers from the label using the difference in specific gravity therebetween, since the latter label has a specific gravity much greater than 1.0 and cannot be significantly separated from such PET bottles.